1. Field of the Invention
The present invention relates to a method of manufacturing semiconductor integrated circuits having thin film transistors formed on an insulating surface. In the context of the present invention, the term xe2x80x9cinsulating surfacexe2x80x9d means an insulating substrate, an insulating film formed thereon, or an insulating film formed on a material such as a semiconductor and metal. More particularly, the present invention relates to semiconductor integrated circuits which employ a metal material mainly composed of aluminum as the material for gate electrodes and gate lines, such as active matrix circuits used for liquid crystal displays.
2. Description of the Related Art
Thin film transistors (TFTs) have been manufactured using a self-alignment process with the aid of single crystal semiconductor integrated circuit techniques. According to this process, a gate electrode is formed on a semiconductor film through a gate insulation film and impurities are introduced into the semiconductor film using the gate electrode as a mask. Impurities can be introduced using methods such as thermal diffusion, ion implantation, plasma doping, and laser doping.
Conventionally, the gate electrodes of TFTs have employed silicon having conductivity enhanced by doping with the aid of single crystal semiconductor circuit techniques. This material has high heat resistance characteristics and hence has been an idealistic material in a case wherein a high temperature process is performed. However, it has been recently found that the use of a silicon gate is not appropriate.
The first reason is that silicon has low conductivity. This problem has not been significant in devices having a relatively small surface area. However, it has become significant because the increasing size of liquid crystal displays has resulted in increases in the size of active matrix circuits and design rules (the widths of gate lines) left unchanged.
The second reason is that as the size of devices has been increased, it has become necessary to switch the material for substrates from expensive materials having high heat resistance characteristics such as quartz and silicon wafers to less expensive materials having lower heat resistance characteristics such as the glass available from Corning Corp. under product No. 7059 and the borosilicate glass available from NH Technoglass Corp. under product NA-35, NA-45 etc. Such materials have not been appropriate as materials for substrates because the formation of silicon gates involves a heating process at 650xc2x0 C. or higher.
In view of such a problem, it has been necessary that the silicon gates must be replaced by aluminum gates. In this case, although pure aluminum may be used, a material such as silicon, copper and scandium (Sc) is added in a small amount because pure aluminum exhibits extremely low heat resistance characteristics. Even with such an additive, aluminum still has the problem with heat resistance characteristics. Therefore, for aluminum gates, it has not been possible to use a thermal annealing process to activate impurities after a doping process such as ion implantation utilizing accelerated ions, and optical annealing utilizing, e.g. laser irradiation has been employed for such a purpose. Even in the latter case, severe limitations have been placed on the intensity of the light to irradiate the aluminum gates in order to prevent damage to the gates by the laser irradiation.
Aluminum itself reflects light in a wide range of wavelengths including ultraviolet rays and infrared rays if it has a mirror surface. However, the use of aluminum has not been appropriate, for example, where flash-lamp annealing is employed. The reason is this process involves irradiation for a long time which results in a rise in the temperature of a silicon film caused by the light absorbed by the silicon film or the like and the temperature rise transferred to the aluminum as a result of thermal conduction causes melting and deformation of the aluminum. The same problem has been encountered in laser annealing and in a method wherein continuously oscillated laser beams are irradiated. When extremely short oscillation pulse laser is irradiated, light absorbed by a silicon film operates on only annealing for the silicon film, so that the aluminum can be used without increasing the temperature of the aluminum.
FIGS. 4A to 4E show steps for manufacturing thin film transistors having an aluminum gate based on the above-described considerations. First, a backing insulation film 402 is formed on a substrate 401 and island-like crystalline semiconductor regions 403 and 404 are formed thereon. An insulation film 405 which serves as a gate insulation film is formed to cover those regions. (FIG. 4A.)
Then, gate electrodes/gate lines 406 and 407 are formed by using a material mainly composed of aluminum. (FIG. 4B).
Next, impurities (e.g., phosphorous (P) or boron (B)) are implanted on a self-alignment using the gate electrodes/gate lines 406 and 407 as masks according to the ion implantation method, ion doping method, or the like to form impurity regions 408 and 409. In this case, phosphorous is implanted in the impurity region 408 and boron is implanted in the impurity region 409. As a result, the former becomes an N-type region and the latter becomes a P-type region. (FIG. 4C.)
Thereafter, a pulse laser beam is directed from the upper side to active the regions where impurities have been introduced. (FIG. 4D.)
Finally, an interlayer insulator 411 is deposited; a contact hole is formed in each of the impurity regions; electrodes/lines 412 through 416 connected to the contact holes are formed to complete thin film transistors. (FIG. 4E.)
According to the above-described method, however, the boundaries between the impurity regions and regions wherein channels are to be formed (semiconductor regions directly under gate electrodes which are sandwiched by the impurity regions, e.g., the region indicated by 410 in FIG. 4D. are electrically unstable because they have not been subjected to a sufficient treatment during processing. It has been found that those regions create problems such as an increase in a leak current, thereby reducing reliability after use for a long period.
Specifically, as apparent from the processing steps illustrated, neither introduction of impurities nor laser irradiation takes place once a gate electrode is formed. Therefore, substantially no change occurs in the crystallinity of the region where a channel is to be formed.
On the other hand, impurity regions adjacent to a region wherein a channel to be formed initially have the same crystallinity as that of the region wherein a channel is formed. However, the crystallinity is decreased by the introduction of impurities. Although the impurity regions are repaired by a laser irradiation process performed later, it is difficult to obtain the initial crystallinity. Especially, the areas of the impurity regions which are adjacent to the active region can not be sufficiently activated because such areas are not likely to be irradiated with laser light. Specifically, since the crystallinity is discontinuous between the impurity regions and the active region, a trap level or the like produces easily. Especially, when impurities are introduced using a method wherein accelerated ions are applied, impurity ions are dispersed into the area under the gate electrode portion and destroy the crystallinity in that area. It has not been possible to activate such an area under a gate electrode portion using a laser beam or the like because the gate electrode portion blocks the beam.
This equally applies to the gate insulation film. Specifically, while the gate insulation film above the region wherein a channel is to be formed remains in the initial state, the gate insulation film above the impurity regions undergoes great changes during steps such as introduction of impurities and laser irradiation. As a result, many traps occur at the boundaries between those regions.
One possible solution to this problem is to perform activation by irradiating the substrate on the rear side thereof using a laser or the like. According to this method, since the gate lines are not blocked from the light, the boundaries between the active regions and impurity regions are sufficiently activated. In this case, however, the material of the substrate must transmit light. Since most glass substrates can not easily transmit ultraviolet rays having wavelengths of 300 nm or less, for example, a KrF excimer laser (having a wavelength of 248 nm) that excels in mass productivity can not be used.
Further, during the laser irradiation step as described above, aluminum is heated to a high temperature, although only instantaneously. This has resulted in abnormal growth of aluminum crystals (hillock). Especially, abnormal growth in the vertical direction can cause a short circuit between the aluminum crystals and wiring above them.
When ion doping is carried out to dope impurities, another problem arises. Ion doping is a method wherein a gas including impurities for doping (e.g., phosphine (PH3) if phosphorous is to be doped and diborane (B2H6) if boron is to be doped) is subjected to electrical discharge and resulting ions are taken out and emitted using a high voltage.
This method is simpler compared to ion implantation and is suitable for processing a large surface area. According co this method, however, various ions are emitted because mass separation is not performed. Especially, a very large amount of hydrogen ions are emitted both in atomic and molecular states. If such hydrogen ions exist in the gate insulation film in the vicinity of a gate electrode (the gate insulation film above the region 410), fluctuations in characteristics can be caused when a voltage is applied. Especially, the method shown in FIGS. 4A to 4E has had a problem in that hydrogen implanted in a gate electrode can not be sufficiently removed.
The present invention confronts the above-described problems, and it is an object of the present invention to provide a method of manufacturing a reliable thin film transistor by achieving continuity in crystallinity between active regions and impurity regions and to provide a high performance thin film semiconductor integrated circuit by integrating such highly efficient thin film transistors.
The present invention solves the above-described problems by activating regions wherein channels are to be formed in addition to impurity regions and gate insulation film using a thermal annealing or an optical annealing process wherein those regions are irradiated by optical energy emitted by an intense light source such as a laser or a flash lamp.
The present invention employs a basic configuration as follows. First, a material which serves as a mask for the formation of impurity regions is formed on island-like crystalline semiconductor regions and, thereafter, doping impurities are introduced into semiconductor films by means of ion doping or the like using the mask. Preferable materials for the mask include insulating materials such as organic materials, e.g., polyimide and silicon-containing materials, e.g., silicon oxide and silicon nitride and conductive materials such as metals, e.g., aluminum, tantalum, and titanium and conductive metal nitrides, e.g. tantalum nitride and titanium nitride. If it is desirable to prevent the semiconductor regions from directly contacting the mask, a film of silicon oxide or silicon nitride may be formed between them.
Next, the mask is removed to form an insulation film which serves as a gate insulation film. Thereafter, a thermal annealing process or an optical annealing process is performed not only to activate the doped impurities but also to improve the characteristics of the interface between the gate insulation film and the regions wherein channels are to be formed and the characteristics of the boundaries between the regions wherein channels are formed and the impurity regions. This may be achieved using an optical annealing process or a thermal annealing process alone or using a combination of optical and thermal annealing processes.
In the thermal annealing process, the annealing temperature is set at 650xc2x0 C. or less. If the optical annealing process is performed using a laser, it is possible to use various excimer lasers including KrF lasers (wavelength: 248 nm), XeCl lasers (wavelength: 308 nm), ArF lasers (wavelength: 193 nm), and XeF lasers (wavelength: 353 nm), Nd:YAG lasers (wavelength: 1064 nm) and second, third and fourth harmonics thereof, carbon dioxide lasers, argon ion lasers, copper vapor lasers, and the like.
Incoherent light sources are inexpensive and readily available. For example, xenon lamps, krypton arc lamps, halogen lamps and the like may be used. Optical processing using such light sources is not limited to irradiation of the semiconductor region on the upper surface thereof but may be performed by irradiating the semiconductor region on the bottom surface or on both upper and bottom surfaces thereof.
Such a thermal annealing process or optical annealing process can be effectively performed in an atmosphere containing halogen elements (atmosphere containing hydrogen chloride, chlorine, ethylene trichloride, hydrogen fluoride, fluorine, nitrogen trifluoride, and the like) or an oxygen atmosphere (atmosphere containing oxygen, nitrogen oxides of various types, ozone, and the like).
A gate electrode may be formed so that it is offset from impurity regions or so that it overlaps with the impurity regions. An offset gate will reduce the leak current of the TFT. However, since an offset gate has small amount of current when the TFT is turned on, it has the disadvantage of low operating speed. For this reason, offset gates are normally used only in pixel switching TFTs and sampling TFTs of an active matrix circuit, whereas gates which slightly overlap with impurity regions are used for other logic circuits. Although an overlap gate is not suitable for high speed operations because it has a parasitic capacity, it has no problem in driving an active matrix circuit.
The upper and side surfaces of all or some parts of gate electrodes and gate lines thus formed are anodized to form aluminum oxide films having high voltage withstand characteristics which prevent the gate electrodes and gate lines from shorting with wiring in the layer above them. The formation of such anodic oxide films is effective for preventing interlayer short circuits especially in an active matrix circuit wherein may lines intersect with each other. Further, since aluminum oxide has a high dielectric constant, it can form a capacitor with a line in the layer above it. Although anodization is normally performed in an electrolytic solution on an electrochemical basis, it goes without saying that it may be performed in a low pressure plasma atmosphere as in the well-known plasma anodization process.
According to the present invention, the gate electrodes and gate lines have not been formed when a thermal annealing or an optical annealing is carried out to activate impurities which have been doped. This relaxes the tolerance for a thermal annealing or an optical annealing when compared to the conventional doping on a self alignment as shown in FIGS. 4A to 4E. For example, the present invention allows the use of a thermal annealing or a flash lamp annealing unlike the prior art.
In a thermal annealing process, since the impurity regions, the regions wherein channels are formed, and gate insulation films are uniformly heated, no discontinuity occurs at the boundaries between them. Similarly, no discontinuity occurs in the case of an optical annealing because there is no gate electrode which blocks light.
An optical annealing or a thermal annealing provides an effect of replacing residual hydrogen atoms in the gate insulation films and semiconductor regions when performed in an halogen atmosphere or an oxidizing atmosphere. Strong electrical fields are generated at the gate insulation films and regions wherein channels are formed. If hydrogen atoms exist in the form of a silicon-hydrogen or oxygen-hydrogen combination in such electrical fields, the electrical fields decouple the hydrogen atoms, thereby causing changes in the characteristics of those regions over time. When a halogen, especially fluorine or chlorine, exists in those regions instead of hydrogen, the characteristics of those regions are stable. This is because a halogen is very strongly coupled with silicon or oxygen and is not easily decoupled.
Further, when an ion doping process is used to dope impurities, the ion doping is carried out in the absence of the gate insulation films. As a result, hydrogen ions are not implanted in the gate insulation films, and this provides very stable characteristics.
In addition, in a circuit having intersecting lines, the anodization of the upper and side surfaces of the gate electrodes prevents the gate electrodes from shorting with lines in the layer above them due to the occurrence of hillock. Especially, aluminum is characterized in that it provides an anodic oxide film having high voltage resistance which have not been achievable with a conventional silicon gate.